1. Field of the Invention
The present invention relates to polymeric flame retardants and, more particularly, to cyclic imide or cyclic imide-forming polymers or copolymers bearing halogenated aromatic substituents and methods for preparing the same. Further, the present invention is directed to methods for inhibiting ignition of polymers or polymer blends under conditions which promote combustion by inclusion of these polymeric flame retardants.
2. Description Of The Prior Art
Three factors typically affect the performance of a flame retardant additive in a polymer composition: (1) the intrinsic activity of the flame retardant on a molar basis; (2) the compatibility or solubility of the flame retardant in the polymer system; and (3) the ability of the flame retardant to remain in the polymer system. Of these factors, the third is generally most problematic. Conventional flame retardants tend to volatilize or migrate from the polymer system, thereby lowering the flame retardancy of the system.
One popular approach to solving the volatility and migration problems of the flame retardants has been to use brominated or chlorinated monomers in the polymerizations or copolymerizations. French Demande 2,406,644 (Chem. Abst. 91, l58528s), for example, discloses the preparation of fire resistant foams by foaming maleic anhydride-styrene-(2,4,6-tribromophenoxy)ethyl methacrylate copolymer or ar-bromostyrene/maleic anhydride copolymer. German Offenlegungsschrift 2,659,174 (Chem. Abst. 87, 118404j) discloses flame retardant copolymers of styrene, maleic anhydride and bromophenyl or bromoneopentyl (meth)acrylate. Czech Patent 171,593 (Chem. Abst. 89, 08730q) discloses halo derivatives of N-phenylmaleimide which are useful as comonomers in the preparation of polymers with reduced combustibility.
Copolymers containing various ratios of bromostyrenes and styrene are also well known in the literature and have been used in blends with poly(phenylene oxide) to improve the fire resistance (see French Demande 2,483,443, (Chem. Abst. 96, 163728q) for example). Copolymers of styrene and chlorostyrenes have also been blended with poly(phenylene oxide) to increase the fire resistance of the polymer blends (see Fried, J. R., et al., Macromolecules, 11:150 (1978)). Rubber modified styrene-bromostyrene copolymers are also known (see, for example, Japanese patent publication 80,120,654 Kokai Tokyo Koho (Chem. Abst. 94, 85l28m)) and have been blended with poly(phenylene oxide) to increase fire resistance.
A problem arises, however, when halogenated monomers are incorporated into the copolymer. The physical properties of the halogenated copolymer are changed relative to the unhalogenated homopolymer. Further, in many cases, the copolymer has limited or no compatibility with the polymer in which it is blended. Consequently, the desirable physical properties of the polymer blend are destroyed or degraded by incorporation of the halogenated monomer (see Ten Brinke, G., et al., Macromolecules 16:1827 (1983)). In addition, the use of a halogenated monomer is often unsatisfactory in commercial practice because it requires, as a first step, the halogenation of the monomer. This separate step is costly and inconvenient.
Flame retardant compositions containing styrene-N-halogenated maleimides are known and are described generally in U.S. Pat. Nos. 4,508,883; 4,604,422; and 4,609,711, issued to U. E. Younes. U.S. Pat. No. 4,508,883, for example, discloses a flame retarding additive prepared by copolymerizing a brominated styrene and an N-(brominated phenyl)maleimide.
U.S. Pat. No. 4,604,422 discloses a moldable composition which comprises random N(brominated or chlorinated phenyl)maleimide containing copolymers. The copolymers are prepared by copolymerizing an N-(brominated or chlorinated phenyl)maleimide with a monomer, such as styrene or substituted styrene, and, if desired, an alpha, beta-unsaturated dicarboxylic anhydride. Rubber may be grafted onto these polymers during the preparation. The copolymers are blended with polycarbonate to produce flame retardant compositions.
U.S. Pat. No. 4,609,711 discloses a moldable composition which comprises a random N-(halogenated phenyl)maleimide containing copolymer and a flame retardant synergist. The copolymers are prepared by copolymerizing an N-(brominated or chlorinated phenyl)maleimide with a monomer, such as styrene or substituted styrenes and, where desired, a suitably substituted maleimide.
The compounds and compositions of the Younes patents are not known to be commercially available. The flame retardant compounds of the Younes patents are prepared by first reacting maleic anhydride with the corresponding halogenated aniline in the presence of zinc chloride at 139.degree. to 148.degree. C. for about 2 hours and isolating the N-(brominated or chlorinated phenyl)maleimides by extraction into xylene. The maleimides are subsequently copolymerized with vinyl monomers to form the flame retardant polymers. The N-(brominated or chlorinated phenyl)maleimide containing copolymers are not prepared by reaction of the halogenated aniline with the corresponding anhydride containing polymer. This is due to the poor reactivity of halogenated anilines with the anhydride containing copolymers (especially when there is more than one halogen on the aniline).
Dow Chemical Company avoided the problem of the poor reactivity of the halogenated anilines by using brominated primary aliphatic amines. U.S. Pat. No. 4,544,682, for example, discloses flame retardant polymeric compositions containing copolymers of a styrenic monomer and a maleimide having a bromine substituted aliphatic hydrocarbon radical appended to the nitrogen atom. The flame retardant compositions are formed by melt blending copolymers of styrenic monomers and maleic anhydride with brominated primary aliphatic amines. The preferred brominated primary aliphatic amine was 2,4,5-tribromobenzylamine, which is not known to be commercially available.
Halogenated aromatic compounds containing a hydrazide functionality are known in the art. For example, N-(halophenyl)-N'-aminooxamides are known and described generally in Chemical Abstract registry numbers (RN) 17739-01-4 (3,4-dichloro), RN 60199-81-7 (4-bromo), RN 53117-25-2 (2-chloro), RN 53117-26-3 (3-chloro), RN 53117-27-4 (4-chloro), and RN 53117-31-0 (3,5-dichloro). However, none have been reacted with anhydrides, either monomeric or polymeric.
In addition to the flame retarding properties of the polymeric flame retardants of the present invention, the acylhydrazido linkage created by the reaction of the hydrazide group of the halogenated aromatic group and an anhydride group of the polymer or copolymer to form the polymeric flame retardant contributes permanent heat stabilizing and metal deactivating properties to the polymer. These properties arise from the structure of the group formed during attachment, i.e., acylhydrazido and N-(acylamino)imide functional groups.
Acylhydrazido functions, for example, are known to be particularly useful antioxidants (see, e.g., U.S. Pat. No. 3,639,334) and also act as metal deactivators by chelating metal ions, such as copper ions, (U.S. Pat. Nos. 4,147,689; 4,465,571 and 3,887,518). N-(aroylamino)imides are also known stabilizers of polyolefins against degradation by heavy metals (see U.S. Pat. No. 3,956,331).
Nothing in the prior art, however, discloses or suggests attaching flame retardant groups to anhydride-containing polymers or copolymers by hydrazido linkages between the flame retardant groups and some or all of the anhydride groups. Likewise, nothing discloses or suggests any flame retardant polymers where halogenated aromatic groups are attached to polymers by pendant acylhydrazido or N-(acylamino)imide groups.